Permanent magnet and method of making same



Patented June 10, 1941 UITED MS PATENT rrica Gottfried Bruno Jonas,Elndhoven, Netherlands, assignor, by mesne assignments, to HartfordNational Bank and Trust Company, Hartford,

Conn., as trustee No Drawing.

Application February 13, 1937, Se-

rial No. 125,654. In Germany March 17, 1936 3 Claims.

My invention relates to permanent magnets, and to a method of producingsame.

Although Ni-Ti alloys, as well as Ni-Co-Ti alloys are used for permanentmagnets, the desired magnetic properties are generally not obtained.

manent magnet are not obtainable at all, Even if such alloys areimproved by adding substances such as Cr, Mo, V, Al, As, or W, and areheattreated in the same manner, only in exceptional cases are thedesired results obtained.

The object of my invention is to overcome the above difiiculties and toproduce a permanent magnet having high m agnetic properties.

In accordance with my invention, I select a definite group of alloysfrom the entire known and practically infinitely large region of theabove-mentioned magnet steel alloys, and heattreat these alloys in sucha manner that, after magnetisation, particularly satisfactory values ofremanence and coercive force are obtained.

More particularly, I use a steel alloy comprising 13-22% nickel, 8-30%cobalt, 8-0.5% titanium, 4-11% aluminium, up to 1.5% of impurities, andthe remainder iron. In some cases I also use from 0.5-5% copper.

I form these constituents into an alloy by melting or sintering same,whereupon I cool the alloy from a temperature of about 1200 down toabout 650 0., either directly from the melt or after the alloy has beenre-heated to a high temperature, at a speed determined by thecomposition of the alloy, and at an average rate less than 8 C. persecond. Thereupon I further cool the alloy to room temperature in anydesired manner, for example in air.

The magnet bodies can be produced by casting I the alloy, or byintermixing the constituents in a finely divided state-preferably in theform of preliminary or intermediate alloys thereof-by compressing themmore or less, and by homogenizing by a sintering process.

Although the addition of copper does not appreciably improve themagnetic properties of the above-mentioned alloys, it generally doesincrease the coercive force which is desirable for many purposes, inspite of the fact that the remanence is reduced.

I prefer to so select the quantities of titanium and aluminum that thetotal amount thereof is about 9 to 15%, preferably about 11%, becausefor the alloys under consideration this percentage produces magnets ofvery -favourable magnetic properties.

desired manner, for example in air.

Instead of adding titanium to form the 8-0.5% titanium content, I mayadd a suitable amount of ferrotitanium which is usually employed forsuch purposes and which may be assumed to contain from about 20 to 40%of titanium depending upon its quality. In the latter case however,those admixtures which in addition to iron and titanium are present inthe ferro-alloy, for example Al, Si, Cu, and Mn, must be taken intoaccount. Particularly the silicon which is nearlyalways present inferrotitanium, should constitute a non-excessive percentage of themagnet alloy, i. e. a percentage less than about 1%, as otherwise itwould deleteriously affect the magnetic properties of the alloys. Themanganese present in the ferro-alloy does not substantially alter themagnetic properties of the alloys. In the same way I may add a suitableamount of ferrocobalt to form the 8-30% cobalt content.

I prefer not to use simultaneously in the same alloy nickel, cobalt andtitanium in the maximum amounts of the above-stated ranges, as with suchan alloy the magnetic maximum values are generally not attained by atechnically simple thermal treatment. With the exception of theabove-mentioned conditions as to the total percentage of titanium andaluminium,

and to the silicon content, there are no further restrictions in thechoice of the percentage of the various components as long as they arewithin the prescribed ranges.

Magnet steel alloys obtained by the process of the invention may have,after magnetisation, a remanence of about 6,500-9,500 and a coerciveforce of about 900-300, whereas the majority of the magnets attain aBHmax value of over 1,500,000 gausses and some evenmore than 2,000,000gausses.

The heat treatment according to the invention may be effected indifierent ways, and any magnet steel alloy having a composition withinthe ranges above set forth must be cooled at a definite rate accordingto its composition to obtain the highest values of remanence andcoervice force. I generally carry out the heattreating by heating a bodyformed from an alloy of the above type, to a temperature which is ashigh as possible without reaching the melting point of the alloy-atleast higher than 1150 and then cool the body to a temperature about 650C. at a speed which is less than an average cooling rate of about 8 0.per second, and which has been definitely determined with the aid oftest pieces. I then further cool the body in any By average cooling rateis meant the total decrease in temperature divided by the total time. Inthis hardening process, and particularly with bodies of comparativelylarge size, I prefer.

to use means to cause a very slowly-proceeding hardening, for examplesuch as loamwater, compressed air, wet sand, non-circulating air, drysand, or furnaces heated to a suitable temperature. In some cases, forexample with bodies of medium size and of favorable shape, the bodiesmay be formed by merely pouring the molten alloy into a suitable mold ofdry or wet molding sand in such manner that the particular cooling raterequired is automatically obtained without further measures, and wherebysubsequent hardening may be dispensed with.

In many cases a further improvement in the magnetic properties,particularly in the BHmax, may be obtained by annealing the body afterthe above-mentioned thermal treatment, for example for a few hours, andat temperatures between about 600 to 700 C and then cooling the same inany desired manner.

I have found that the very slowly-proceeding cooling makes the processaccording to the invention very suitable for bodies of larger dimensionsas used in loudspeakers, e. g. for annular magnet-bodies having anoutside diameter exceeding 70 mm., an inside diameter exceeding 40 mm.and a height exceeding 30 mm.

The magnetic properties of magnets produced in accordance with thepresent invention and by prior art method are indicated in Tables I andII below. Table I gives the magnetic properties of magnets produced inaccordance with the invention and for four different alloys as indicatedby Examples I to IV, whereas Table II gives the magnetic properties ofmagnets produced from the alloys of Example I to IV of Table I whenthese alloys are heat-treated by prior art methods.

In the following tables the bodies as originally formed were 10 mms.wide, 32 mms. thick and 100 mms. long, whereas prior to the annealingthe bodies were divided into pieces having a width of 10 mms., athickness of 12 mms. and a length of 30 mms. In the tables, Br denotesthe residual flux density in gausses, whereas He indicates the coerciveintensity in gausses.

TABLE I According to the invention TABLE II According to a usual priorart process Rapid cooling (from 1253 G for one minute in bath of Aftermolten tin annealing at 650 C. for about and 6% hours further coolingComposition of alloy to room temperature From Tables I and II it is seenthat in all the examples the magnets produced according to the inventionhave He values greatly exceeding the He values of magnets produced bythe prior art method, whereas without annealing the Br values exceedthose of the prior art magnets.

It should be noted that also in Example I, ferrotitanium is used fromwhich the content of titanium indicated has been calculated. Besides,the aluminium in the ferrotitanium is present in the amount giventherefore.

The following two examples illustrate the application of the inventionto magnets for use in loudspeakers.

Example V.An annular magnet having an outside diameter of 74 mms., aninside diameter of 4'7 mms. and a height of 37 mms. (which is a sizecommonly used in present-day electrodynamic loudspeakers) was cast froma composition obtained by melting together an alloy containing about 16%Ni, 25% Co, 12.5% ferrotitanium (equivalent to about 3% Ti), 6.8% Al,

r 2.5% Cu and the remainder substantially iron.

Cooling in sand mold at rate of about 1 0. per Ex. Composition of alloysec.

After re-heating After tel, to 1250 C. and being per see. more B, H, B,H, B, H, B.- H.

Rate of cooling (about 3 0.

per sec.) 16% Ni, 25% (-0, about 4% Ti, 7% Al,

remainder Fe 8000 440 8000 592 9900 350 9000 503 Rate of cooling about 30. per sec. ll 16% Ni, 25% Co, 13% FcrrotiL, (about 2.8% T1), 6.5% Al,1% Cu 9000 400 8550 500 9700 324 9200 472 Rate of cooling about 3 0. persec. III..- 10% Ni, 25% Co, 19% Ferrotit., (about 4% Ti) 5.1% Al,remainder Fe 8050 476 7800 032 9200 344 8550 524 Rate of cooling 7 0.per sec. IV. 17% Ni, 10% Co, 10% Ferrotit, (about 4% Ti), 7% Al,remainder Fe 6000 288 0050 320 7850 280 7800 350 in air for about 4.75minutes to about 650' 0. Thus the average cooling rate was about 1.7 persecond. After being magnetized, the resulting magnet had a BI value ofabout 9,000 gausseaand an H6 value of about 470 gausses.

Example VL-A ring of the same size as that of Example V was cast from acomposition obtained by melting together an alloy containing about 16%Ni, 25% Co, 20% ierrotitanium (equivalent to about 4.5% Ti) 5% Al andthe remainder substantially iron, After being re-heated, the

casting was cooled from a temperature or about 1250' C., in the presenceof air in about 4 minutes and 10 seconds down to a temperature of about650 C., i. e. at an average cooling rate of about 2 C. persecond. Aftermagnetisation the resulting magnet had aBivalue 01' about 8,600 gaussesand an He value of about 500 gausses.

While I have described my invention in connection with specific examplesand applications I do not wish to be limited thereto, but desire theappended claims to be construed as broadly as permissible in view of theprior art.

What I claim is: 4

1. A permanent magnet comprising approxi mately 16% nickel,approximately 25% cobalt, approximately 2.8-4.5% titanium, approximatelymately 1.7-3.0 C. per second, and cooling the' alloy to roomtemperature. I

3. Amethod of making a permanent magnet body comprising the steps offorming an alloy of approximately 16% nickel, approximately 25% cobalt,GPPI'OXIIIlfltC1Y1-2.8-4.5% titanium, approximately 5.1-7.0% aluminum,and the reminder substantially iron, cooling the alloy from atemperature lying between about 950 C. and

, the melting point to a temperature of about 650 C. at a cooling ratebetween approximately 1.7- 3.0 C. per second, annealing the alloy at atemperature 01' about 600 C. to 700 C., and cooling the alloy .to roomtemperature.

GQTIFRHH) BRUNO JONAS.

